1. Field of the Invention
The present invention relates to a multimodal polyethylene copolymer resin composition comprising a lower molecular weight ethylene polymer (LMW) fraction and a higher molecular weight ethylene polymer (HMW) fraction, and to a process for preparing the above mentioned multimodal polyethylene copolymer resin composition, and to a polymeric product comprising and/or made from the above mentioned multimodal polyethylene copolymer resin composition.
The term “multimodal” means herein, unless otherwise stated, multimodality with respect to molecular weight distribution and includes also bimodal polymer.
2. Description of the Prior Art
Usually, a polyethylene comprising at least two polyethylene fractions, which have been produced under different polymerization conditions resulting in different (weight average) molecular weights and molecular weight distributions for the fractions, is referred to as “multimodal”. The prefix “multi” relates to the number of different polymer fractions present in the polymer. Thus, for example, multimodal polymer includes so called “bimodal” polymer consisting of two fractions. The form of the molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as a function of its molecular weight, of a multimodal polymer will show two or more maxima or is typically distinctly broadened in comparison with the curves for the individual fractions. For example, if a polymer is produced in a sequential multistage process, utilizing reactors coupled in series and using different conditions in each reactor, the polymer fractions produced in the different reactors will each have their own molecular weight distribution and weight average molecular weight. When the molecular weight distribution curve of such a polymer is recorded, the individual curves from these fractions form typically together a broadened molecular weight distribution curve for the total resulting polymer product.
Traditionally, a cascade of reactors is used wherein different reaction conditions are applied during the catalytic polymerization in order to obtain a broad multimodal molecular weight distribution. By using a cascade of reactors it is possible to produce an ethylene polymer resin composition comprising a lower molecular weight (LMW) fraction and a higher molecular weight (HMW) fraction. However, when the higher molecular weight fraction is produced, the comonomer concentration in the higher molecular weight part decreases with increasing molecular weight. This is typical especially for polymers produced using Ziegler Natta catalysts.
Furthermore, the higher molecular weight (HMW) fraction typically has also an influence on the mechanical properties of a final polymer so that e.g. with said high molecular weight (HMW) fraction i.e. environmental stress crack resistance (ESCR) can be controlled.
Multimodal polyethylene compositions produced in a multistage process and comprising at least a low molecular weight (LMW) fraction and a high molecular weight (HMW) fraction may sometimes comprise polymer particles of “very high molecular weight”,i.e. gels, defined e.g. as the “white spots” in the prior art literature, which particles can cause inhomogeneities in the composition and the end-products produced thereof.